Conventionally the intake tracts of a multi-cylinder engine are brought together to a common plenum chamber, each tract opening into the plenum chamber at a point which is designed to experience the minimum flow interference from the other tracts. The tracts function as tuned lengths for which there is one engine speed where the torque is enhanced because of resonance of the pressure wave excitations within the intake system. More specifically, when the inlet valve starts to open, at the beginning of a four stroke cycle, the combustion chamber is still under pressure from the exhaust stroke of the previous cycle and a pressure wave passes out of the intake port and propagates at the speed of sound along the intake tract. This wave travels the entire length of the intake tract until it reaches the plenum chamber at which point it is reflected with opposite phase. A further three traverses of the entire length of the intake tract and two reflections at the open and closed ends of the tract are necessary before the wave again reaches the intake port as a positive pressure wave.
Thus, with a fixed length tract, there are engine speeds at which the reflected pressure wave increases the charge density at the instant when the inlet valve closes so as to improve volumetric efficiency but other speeds when the wave arrives as a negative pressure tending to reduce volumetric efficiency.
To obtain an improvement at low engine speeds, a long tract length is required but this has the disadvantage of increasing the package size and the resistance to flow at high engine speeds.
Variable length manifolds have been proposed to avoid the performance degradation at certain engine speeds but these do not provide the solution to the packaging problems in designing a manifold which will resonate at low engine speeds.